A relaxation oscillator, such as a relaxation voltage/current controlled oscillator, is an oscillator circuit which has its frequency of oscillation determined by the time to charge and/or discharge a capacitor (or other reactive element) to a threshold level.
In U.S. Pat. No. 4,977,381, Main discloses a relaxation oscillator in which a capacitor is placed in a differential configuration between a first and a second current supply means. The direction in which the capacitor is charged or discharged is controlled by a differential pair of transistor switches which are turned on and off by a bistable circuit element such as a latch. A control signal which gates the bistable element is inverted when the voltage across the capacitor is at opposite polarities of a particular threshold. The bistable element eliminates sudden voltage jumps at switching times and thereby avoids saturation of the collector-emitter conduction paths of the transistor switches when they are suddenly turned on. A circuit responsive to the dynamic terminal voltages of the capacitor controls either the first current supply means or the second current supply means to maintain the DC (direct current) voltage of the capacitor terminals at a constant, predetermined value. As a result, the slew rate of the voltages at the terminals of the charging capacitor remains substantially constant for a given frequency of operation.
While the relaxation oscillator disclosed by Main desensitizes the oscillator to the effects of the inherent stray capacitance and improves the accuracy of the output frequency, it is susceptible to certain drawbacks. Generally, in relaxation oscillator circuitry the charging current through the capacitor must be reversed once the capacitor voltage reaches a threshold value. However, there is a delay associated with this toggling. During this delay, the capacitor continues to charge, and its voltage exceeds the threshold level during the period until the direction of current is effectively switched. As a result, the frequency of oscillation is also reduced.
As the frequency of oscillation increases, this delay becomes more and more problematic and causes the frequency to cease to depend linearly on the charging current. A further consequence of this non-linearity is that the maximum frequency, and therefore the frequency range, for practical operation of the relaxation oscillator is reduced. Also, since the delay is dependent on temperature and process, the oscillator becomes overly sensitive to variations in these factors.
Yet another drawback of prior art relaxation oscillator circuits is that they require high quality complementary transistors.
It is therefore an object of the present invention in one aspect to provide an improved relaxation oscillator with an extended linear frequency range.